Transmission of short optical pulses is emerging as the best choice in high bit-rate and/or long-distance systems. However, the pulses suffer from nonlinear intra-channel effects, which weaken the performance thereby reducing the distance or decrease the bit-rate.
Long-haul transmission of information with optical fibers and in-line optical amplifiers, using digital on/off transmission format, suffers from two main impairments. One is the presence of the amplified spontaneous emission (ASE) noise of the amplifiers. A way of combating ASE noise is the use of high power signals, in which ones are represented by pulses with energy high enough to be faithfully detected at the receiver side. The second impairment is the signal distortion caused by optical nonlinearity, chiefly the Kerr effect. Optical nonlinearities can be counteracted by reducing the signal power as much as possible. The signal power that permits the achievement of the maximum distance can then be determined by a compromise between the two conflicting requirements of signal power and optical nonlinearity. Usually, it is determined by increasing the power of the signal up to a point where optical nonlinearity increases so much that it distorts the signal beyond an acceptable level. At the optimum power, the system is simultaneously limited by amplified spontaneous emission noise and by the nonlinearity. Indeed, if the transmission system were limited only by the spontaneous emission noise, increasing the power would permit an increase in the distance and if the transmission system were limited only by the optical nonlinearity, reducing the power would permit an increase in the distance.